It must be, there are at least 3 of them living around the pond and I often see them in the water hunting newts.
Or house is next to the river on what was once a water meadow, it's still quite wet in places with drainage ditches and ponds, so it's not surprising there are lots of grass snakes around.
You're so lucky to have a wild snake in your garden- I've never so much as caught a glimpse of one, even down here in Cornwall. I've never seen an adder either. There is a grass snake at College (I'm an animal care student), in a collection of native cold-blooded animals.
The first summer we were here I witnessed several hatchling grass snakes wriggling out of the compost heap, I don't think it had been used for some time, but now we use it a lot so I assume the snakes don't lay there any more. I should really start a second heap and just leave it for them.